In a plasma processing apparatus for performing plasma etching or the like on a target substrate, the processing is often performed while a temperature of the substrate being changed stepwisely. FIG. 6 is a chart diagram showing a stepwise temperature control in performing processing in a same processing chamber while etching conditions (gas types, temperatures, pressures and the like) being changed. In order to perform the stepwise temperature control, the temperature needs to be controlled at a high speed. For example, the time required for each step should be less than about 30 seconds. During the temperature control, the substrate temperature is increased or decreased compared to the previous step. In any cases, it is required to shorten a period of time for increasing or decreasing the substrate temperature.
Conventionally, the substrate temperature is changed by heating and cooling a susceptor which mounts thereon a substrate and functions as a heat exchanger plate. The susceptor is cooled by circulating a coolant therein, and is heated by a heater, e.g., a thermoelectric element module or the like, provided directly under the substrate. In other words, it is often the case that the susceptor is heated and cooled by separate units.
Meanwhile, in the plasma processing apparatus, a high frequency power for plasma generation is applied and it is required to prevent an RF (radio frequency) from leaking from wire lines of the thermoelectric module (heater). To do so, an RF filter needs to be provided at a wiring system of the heater. However, this makes the equipment complicated and increases the cost, which is not desirable. Further, even if the RF filter is added, it is not possible to completely suppress RF noise and RF power loss. Furthermore, the filter needs to be changed depending on the RF frequency and the RF power. Besides, although ceramic is generally used as a heat source of a heater, ceramic can be damaged by rapid thermal contraction. For that reason, the rapid temperature increase by the heater is limited.
The following Patent Documents describe techniques in which the susceptor (heat exchanger plate) is cooled and heated by heat exchange of a heat transfer medium (e.g., cooling water and heating water) (Patent Documents 1 and 2). That is, the temperature of the susceptor can be arbitrarily changed by providing, instead of a heater, a cooling circulation circuit and a heating circulation circuit of the heat transfer medium and controlling the flow rates or the mixing ratio of the heat transfer medium supplied from both circuits to the susceptor.
[Patent Document 1] Japanese Patent Laid-open Publication No. 2001-134324
[Patent Document 2] Japanese Patent Laid-open Publication No. H7-271452
[Patent Document 3] Japanese Patent Laid-open Publication No. 2006-156938
In the above-described method for heating and cooling the susceptor by circulating the heat transfer medium therein, the entire susceptor has the same temperature and, thus, the thermal conductivity between the substrate and the susceptor cannot vary depending on portions of the substrate.
Further, as a diameter of a semiconductor substrate (wafer) increases, variation occurs between heat input and heat output depending on portions of the wafer. Thus, it is difficult to maintain the entire surface of the scaled-up wafer at a uniform temperature.
For example, in the plasma processing apparatus, variation often occurs in the density distribution of the plasma or the temperature distribution of the coolant circulating in the susceptor. Especially, it is difficult to keep the density distribution of the plasma uniform over the entire wafer, so that the balance of heat input and heat output often varies between a peripheral portion and a central portion of the wafer.
In general, the central portion of the wafer is easily cooled, whereas the cooling of the peripheral portion of the wafer is retarded. Therefore, the cooling rate in the central portion of the wafer should be differentiated from that in the peripheral portion of the wafer in such a way that the entire wafer can be maintained at a uniform temperature.
In order to differentiate the cooling rates over the wafer portions, there is suggested a method in which the mounting table is divided into a plurality of zones and the amount of cooling gas supplied to the gap between the substrate and the mounting table varies depending on the zones (Patent Document 3).
However, this method is disadvantageous in that the substrate cannot be heated with this method, and also in that the temperature characteristics of the substrate change at the boundaries of the zones. To that end, there arises a need to provide a susceptor capable of varying the cooling rate or the heating rate depending on the wafer portions.